An active matrix substrate used in liquid crystal display devices and the like is equipped with a switching element, such as a thin film transistor (hereinafter “TFT”), provided for each pixel. A TFT that uses an oxide semiconductor layer as an active layer (hereinafter referred to as an “oxide semiconductor TFT”) is known as this switching element. PTL 1 discloses a liquid crystal display device that uses InGaZnO (an oxide constituted by indium, gallium, and zinc) in the active layer of the TFT.
Oxide semiconductor TFTs can operate at a higher speed than amorphous silicon TFTs. Furthermore, since oxide semiconductor films are formed by a simpler process than polycrystal silicon films, oxide semiconductor films are applicable to devices required to have a large area. Thus, the oxide semiconductor TFTs are expected to be high-performance active elements that can be manufactured with fewer manufacturing steps and less manufacturing costs.
In addition, since the mobility in the oxide semiconductor is high, performance comparable or superior to amorphous silicon TFTs of related art can be obtained even when the size is reduced. Thus, if an active matrix substrate of a liquid crystal display device is manufactured by using oxide semiconductor TFTs, the ratio of the area of a pixel occupied by a TFT can be decreased, and the pixel aperture ratio can be improved. Thus, bright display is possible at a suppressed quantity of backlight, and low power consumption can be realized.
Furthermore, since the oxide semiconductor TFTs have excellent off-leak characteristics, an operation mode with which display is performed at a low image rewriting frequency can also be used. For example, in displaying a still image, operation can be performed so that the image data is rewritten once every second. Such a driving method is called pause drive, low-frequency drive, etc., and can significantly reduce the power consumption of the liquid crystal display device.